Gas generating compositions



2,946,672 Patented July 26, 1960 2,946,672 GAS GENERATING COMPOSITIONS Paul O. Marti, J12, Hammond, Ind., assignor to Standard Oil Company, Chicago, 11]., a corporation of Indiana No Drawing. Filed Apr. 26, 1955, Ser. No. 504,117 10 Claims. (Cl. 52-.5)

This invention relates to a composition for the generation of a gas; and pertains, more particularly, to an improved gas-generating composition comprising ammonium nitrate as the primary gas-producing component of the composition, intimately mixed with a combustible binder material consisting essentially of plasticized polyvinyl chloride. -Such composition is useful for the propulsion of rockets for ground-to-ground missiles, shipto-shore missiles, air-to-ground missiles and air-to-air missiles and. the compositions may also be used as propulsion means in assist take-off of military and commercial aircraft.

Ammonium nitrate is widely used as a component of high explosives, particularly the so-called safe explosives. Even though ammonium nitrate is classified as a high explosive, it is extremely insensitive to ordinary heating and to shock and cannot readily be detonated by the local application of heat or by a blasting cap. Further, when ignited, ammonium nitrate alone does not burn uniformly and has a tendency to go out. In order to improve the burning quality, to increase the sensitivity and to utilize the excess free oxygen available from the decomposition of the ammonium nitrate, and to provide shaped grains suitable for use in rocket motors and assist take-01f motors, combustible binder material is used in the ammonium nitrate composition.

The use of ammonium nitrate-base compositions as solid propellants for rockets and assist take-off units is attractive because of the cheapness and availability of ammonium nitrate, because of the relatively low flame temperature of decomposition of ammonium nitrate, that is, between about 3150 and 2900 F. and because the excess free oxygen available from the decomposition permits the use of oxidizable material to improve the energy available from the decomposition. However, the physical characteristics of ammonium nitrate andgrain material produced therefrom introduce problems with respect to choice of binder material. Thus solid ammonium nitrate exists in different crystalline forms at'different temperatures, the transition from one form -to a different form involving a volume change of the ammonium nitrate. 90 F. and also at about 0 F. involve 3.5% or about 3% increase, respectively. It is, therefore, obvious that an ammonium nitrate-base composition could be seriously affected by storage at temperatures common to such storage conditions.

One requirement for solid propellant suitable for military use is that it be ballistically stable after prolonged storage temperatures as high as about 130 F. oras low as about 50 F. Another requirement is that the grain composition will not shatter or crack after being subjected to alternate high and low temperatures, i.e., 175 F. followed immediately by subjection to a' temperature of 75 F. in a series of at least two repeated cycles and that the burning of the composition following such cycling be uniform and not changed materially I from the burning characteristics of such grain material 2 which has not been subjected to such cycling, through extreme temperatures. The cycling operation causes the .development of internal cracks (fissures) or checking of external surface of the 7 grain which cause non-uniform burning of the grain. Still another requirement of binder material is that'it may be ignited at extremely low temperatures, that is, as low as 75 F. or at .tem-' peratures as high as 165 or 175 F. as well as at inter mediate'ambient temperatures following subjection to hot or cold storage conditions, that is, the grain coma position must be reliable with respect to firing after being Volume changes which occurat about subjected to hot or cold environment. Another requirement is that the grain material be dimensionally stable at'these low and high temperatures, that is, that the shaped grains withstand shattering at low temperatures and distortion at high temperatures when subjected to shoclc An object of this invention is the preparation of a gas-generating composition using ammonium nitrate as the principal gas-generating material. Another object of the invention is .the preparation of a shaped explosive composition (grain), consisting essentially of ammonium nitrate, a binder material consisting essentially of polyvinyl chloride plasticized with at least one nitro aromatic compound and a combustion catalyst, which grain is dimensionally stable and non-fissuring in the temperature range between about 175 and F. Still another object is to produce a gas-generating composi-. tion comprising ammonium nitrate, a binder material consisting essentially of plasticized polyvinyl chloride, said gas generating composition also containing aninorganic combustion catalyst, which gas-generating composition is suitable for use in rockets and assist take-off units. Yet another object is the preparation of a composition which is suitable for use as a binder for a mixture of ammonium nitrate and a combustion catalyst. A further object is to provide an improved grain which is useful as a rocket fuel and as fuel for a jet assisted take-01f unit, which grain is characterized by having a burning rate at 1000 p.s.i., of at least 0.10 inch per second and which is preferably of the order of 0.10 to 0.20 inch per second or more, and of which burning rate the pressure exponent as described hereinbel'ow is below about 0.8 and preferably below about 0.7. Other objects will be apparent as the detailed description of the invention proceeds.

The composition of the invention comprises ammonium' nitrate, a combustion catalyst, and the binder material. The composition contains, on a weight basis, from about 73% to about 88% of finelyground ammonium nitrate, from about 1% to about 5% ofa combustion catalyst and from about 10% to about 25% of combustible binder material of which binder material at least about 15% is a solid polyvinyl chloride resin, usually from about 20% to about 35% by weight, the remaining components of the bindercomprising at least one plasticizer material for the polyvinyl chloride resin; said plasticizer material consisting of a major proportion of at least one nitro aromatic compound as defined hereinbelowI O-thercomponent's such as finely divided nent and surfactant may be added to improve the mixing and molding properties of the composition.

The polyvinyl chlorides of the binder material of this 7 invention are solid vinyl chloride ..homopolymers of the relatively high molecular weight although I may use, as synthetic polymeric materialin the binder composition to'b'e plasticized, interpolymers prepared from vinyl chloride and vinylidine'chloride, which 'interploymers are fabricatedusing'ratios of vinyl chloride to vinylidine chloride, not less than about to 15. I prefer solid 3 vinyl chloride homopolymers. Vinylidine chloride homopolymers are wholly unsuited as synthetic polymeric base material for the binders of this invention.

The solid vinyl chloride homopolymers of this invention are White powders, preferably sufficiently pulverized such that 100% of the polymer passes through a #40 US. Standard mesh screen and at least 50% passes through a #100 mesh screen. These polymers have a specific gravity of about 1.4 at temperatures of 20 C. to 25 C. The specific gravity is determined by weighing an air-free molded cube or disk of the polyvinyl chloride in water and in air at a temperature of 20 to 25 C. and the specific gravity is calculated from the expression where a=weight of specimen in air, b=weight of speci- Specific viscosity 1 where t, is the time of flow of a solution of the sample (approximately 0.4%) in nitrobenzene, using a standard Ostwald pipette having a capillary which will give a flow time to the nitrobenzene solvent of from about 25 to 28 seconds at 20 C. and t is the time of flow of the nitrobenzene solvent, the determination being made at 20 C. In making the determination, 0.48 gram of the dry polymer is dissolved in 90 milliliters of commercial nitrobenzene to make up the test sample. This test sample is aged for a period of from 18 to 20 hours at 70 C. after which it is allowed to cool to 20 C. before making the test. One hundred milliliters of the same batch of nitrobcnzene from which the test sample is made up, is used as the blank for determining the value of t that is, samples and blanks tested at a given time are made up from the same supply of nitrobenzene. The determinations are made in a constant temperature bath at 20 C. with a tolerance of 10.5 C.

Commercial grades of polyvinyl chloride suitable as polymeric base material for the fabrication of the binder are the Geon 100 series as exemplified by Geon 101, Geon 10lE'P, and Geon 121, with a specific viscosity range of 0.52-0.58. Other commercial grades of polyvinyl chloride suitable as binder components of this invention are: PVC 100-1 polyvinyl chloride, Opalon 300, Exon 905 polyvinyl chloride and the QYNA polyvinyl chloride product.

The composition of the gas-producing propellant grain of this invention is substantially as follows:

Percent Ammonium nitrate 73 to 88 Plastic binder 10 to 25 Inorganic combustion catalyst lto 5 Carbon to-5 Surfactant 0 to The plastic binder of this invention consists essentially of 15% to 35% by weight of solid polyvinyl chloride plasticized with 85% to 65% by weight of a plasticizer of which plasticizer a major proportion, that is, at least 60% by weight, consists of nitro substituted aromatic compounds, the remainder of said plasticizer consisting of nonnitro group containing plasticizer material as described hereinbelow.

Ammonium nitrate The term ammonium nitrate as used in this specification 'and in the claims is intended to mean either ordinary commercial grade ammonium nitrate such as 4; t conventionally grained ammonium nitrate containing a small amount of impurities which may be coated with a small amount of moisture-resisting material such as petrolatum or paraffin or to mean military grade ammonium nitrate or mixture of minor amounts (usually less than 10%) of other inorganic nitrates such as sodium nitrate or potassium nitrate with the ammonium nitrate. A mixture of finely ground and unground or coarsely ground ammonium nitrate is preferred. The major proportion of the ammonium nitrate is finely ground in order to reduce the voids to a minimum and hence avoid the use of excess binder material. However, a minor amount of the ammonium nitrate should remain in the unground condition in order to avoid a non-compacting free flowing of the ammonium nitrate in the mixing thereof.

Binder The binder material of the gas-producing propellant composition is a thermoplastic material consisting predominantly of plasticizable solid polyvinyl chloride as described above, said solid polyvinyl chloride being plasticized with at least one plasticizer material, said plasticizer material containing a major proportion, that is, at least 60% by weight of one or more of a group of nitro-substituted aromatic compounds which group in cludes a dinitrobenzene, dinitrotoluene or which may be an ether containing a nitro-substituted benzene nucleus and an aliphatic group attached to the oxygen atom of the ether or which ether may be a diphenyl ether, at least one of the phenyl groups of which is a nitrosubstituted phenyl group.

Since the plasticizer material is the predominant component of the binder, cognizance of stoichiometric oxygen balance is taken in the choice of plasticizer matenal. Thus plasticizers which provide a substantial part of the oxygen requirement are preferred since substantially smokeless combustion of the explosive composition is highly desirable. Suitable plasticizers for the binder compositions may be classified more specifically as others of nitrophenols and as nitromonocyclic aromatics. Esters of polycarboxylic acids such as dioctyl phthalate and dipropyl diglycolate, the esterification product of propyl alcohol with diglycolic acid may be used. In general, simple esters of polybasic carboxylic acids may be used with the nitro-substituted aromatic compounds. Aromatic esters of polyhydric alcohols such as Plastoflex MGB which is a polypropylene glycol dibenzoate may be used as co-plasticizers in amounts usually less than about 40% by weight of the plasticizer material. These non-nitrosubstituted plasticizers furnish good stoichiometric balance to the binder with respect to formation of carbon monoxide and carbon dioxide in the combustion of the grain material. I prefer to use as plasticizer for the solid polyvinyl chloride mixtures a predominance of nitro aromatic compound as described above since the nitro compounds are effective in maintaining high burning rates of the compositions.

The preferred class of nitro-substituted aromatic compounds are dinitro-substituted phenyl nucleus-containing aromatic compounds corresponding to the general formula where R is selected from the class consisting a hydrogen, methyl, phenyl, nitrophenyl, phenoxy, mononitrophenoxy, Z-propeneoxy and alko-xy containing from one to three carbon atoms. Specific examples of the members of this class which are effective for plasticizing the polyvinyl chloride are: m-dinitrobenzene, 2,4-dinitrotoluene, 2,4-dinitrodiphenyl ether, nitrated Do'wtherm A, a mixture of 73.5% diphenyl ether and 26.5% diphenyl, the nitrated diphenyl ether component of which nitrated mixture contains as an average less than three nitro groups per molecule and substantially not more than two nitro groups on any phenyl nucleus. The preparation of the nitrated Dowtherm A is described in the copending application of Wayne A. Proell and Norman J. Bowman entitled Thermoplastic Compositions, filed October 27, 1954, Serial No. 465,132. Other specific examples of nitro others of the above class which are effective plasticizers for the polyvinyl chloride are 2,4-dinitrophenyl allyl ether, 2,4-dinitrophenyl methyl ether and 2,4-dinitrophenyl propyl ether. These are prepared by reacting 2 mols of 2,4-dinitrochlorbenzene per mol of allyl alcohol, methyl alcohol or propyl alcohol, respectively, in aqueous alkaline medium.

Combinations of plasticizers such as dinitrotoluene with 2,4-dinitrodiphenyl ether and combinations of these with dinitrophenyl allyl ether or with dinitrophenyl propyl ether with or without the addition of Plastoflex MGB plasticizer, produce with the solid polyvinyl chloride very satisfactory thermoplastic binder material which, when used in the fabrication of grains containing .the ammonium nitrate, results in the production of grains which exhibit excellent resistance to shattering and deformation when subject to shock at low or high temperatures. Such grains are found not to be altered with respect to burning properties after being subjected to such shock.

The relative amounts of the dilferent plasticizers in the binder material may be varied over a wide range. Thus, the plasticizer may consist entirely of 2,4-dinitrodiphenyl ether or other nitro compounds such asdinitrotoluene, for example 2,4-dinitrotolene, may be used along with the 2,4-dinitrodiphenyl ether in amounts from mere traces of the dinitrotoluene up to about two parts of the dinitrotoluene per part of 2,4-dinitrodiphenyl ether in the binder composition. Likewise, variable amounts of dinitrophenyl allyl ether and/ or dinntrophenyl propy-l ether may be used along with these other nitro compounds. Aromatic esters of polyglycols such as Plastoflex MGB, a polypropylene glycol dibenzoate, may be used in combination with the nitro compound plasticizers. In general, other known plasticizers for polyvinyl chloride containing no nitro group such as the dialkyl phthalates and dipropyl diglycolate should be used only in combination with the nitro compounds as plasticizers for the polyvinyl chloride in the binder material. a I 7 Inorganic combustion catalyst The inorganic combustion catalyst of this invention is selected from the class consisting of ammonium dichromate and Prussian blue catalyst. Certain iron compounds broadly designated as Prussian blue are effective for the combustion of ammonium nitrate grains containing oxidizab'le binder materials. These catalysts are the subject matter of US. patent applications filed by Wayne A. Proell and William G. Stanley, Serial No. 273,564 and Serial No. 288,065, filed May 15, 1952. All of the combustion catalysts disclosed in these applications contain the iron cyanide radical, either ferrocyanide or ferricyanide. In addition to the iron cyanide radical, these catalysts contain a second iron ion which may be either ferric or ferrous. The insoluble Prussian blue is more elfect'ive catalyst at high pressure than the soluble Prussian blue. Thus when operating the combustion chamber containing the solid propellant at pressures between about 500 and 2000 p.s.i., a higher burning rate in inches per second is obtainable when using a given composition containing insoluble Prussian blue as the'catalyst than is obtainable when using for the same composition soluble Prussian blue as the catalyst. Ammoniated insoluble Prussian blue may also be used as inorganic catalyst for the combustion of the grains containing, as binder, plasticized polyvinyl chloride. Such modified Prussian blue catalyst is taught in the application of Wayne A. Proell, Serial No. 288,549, filed May 17, 1952. Mixtures of ammonium dichromate catalyst with the insoluble Prussian blue catalyst may be used. In generalgammonium dichromateis more elfectivefor increasing the burning rate ,of the grain material of this invention and hence the inorganic catalyst should consist at least partially, of am monium dichromate to obtain high burning rates, although as indicated hereinbelow, inorganic catalyst consisting essentially of insoluble Prussian blue, may be used.

Carbon component Finely divided carbon may be added to the gas-producing composition in amounts up to about 5% by weight of the composition, preferably in amounts of 0.5 to 5% by weight of the finishedgrain for the purpoe of improving theburning rate of the composition. Highly adsorptive activated carbons such as Norite and Nuchar well known in the art as activated carbon made from residue organic material, make upone class of effective burning rate components. A second general class of carbon useful for increasing the burning rate of compositions are the carbon blacks, roughly classified as the channel blacks and the furnace combustion blacks. The carton blacks are characterized by low ash content, that is, less than about 0.5%, usually less than about 0.15%, and by having extremely small particle, size, that is,,50 to 5000 A and contain adsorbed hydrogen and oxygen. In order to avoid dusting and to afford convenience in handling, some carbon blacks are formed to the so-called .bead type carbon blacks. The beads are extremely soft and physically unstable and hence become disintegrated during the milling of the composition. Examples of -bead type carbon blacks are Micronex Beads (channel blacks) and Statex Beads (furnace blacks).

A third type of carbon which maybe used in the composition is finely ground petroleum coke, particularly petroleum coke obtained as residue in the pipe-stilling of mid-continent heavy residuums. Such coke usually contains less than about 1% ash and hence, like the carbon blacks, are particularly suitable in gas-producing grains Where it is desirable to keep to a minimum solid inorganic particles in the combustion gas. The coke may be activated by methods well known to theart to improve the efliciency thereofas a burning rate promoter and it is preferably ground to particle size to passthrough a #325 US. Standard sieve to incorporation in the gas-producing compositioni 7 t 1 I Surfactant V V e I have found that the addition of a relatively small amount, that is, up to about 0.5% and'usually from about 0.05% to 0.2%, for. example about 0.1% by weight of a surfactant to the mixture of binder and" ammonium nitrate facilitates contact of these components and improves. the milling characteristicsof the mixture to form the finished homogeneous grain material.

Two broadclasses of surfactants, namely cationic detergents and non-ionic detergents have been found to be particularly effective. 1 Cationic detergents which are effective are organic qu'artenary ammonium halides such as alkenyl dimethyl ethyl ammonium halides and alkyl dimethyl benzyl ammonium halides wherein the alkyl radical (other than methyl-and ethyl) contains from 8 to 20 carbon atoms or the alkenyl radical contains from 8 to 20 carbon atoms. The halogen substituent group is selected from the class consisting of chlorine and bromine. These cationic detergents correspond to the formula CH5 R I wherein R is an alkyl-or an alkenyl group having from 8 to 20icarbon atoms, R' is ethyl or benzyl and X; is either chlorine or. bromine. An example of such a'cationic surfactive agent effective for improving the milling characteristics of the mixtures of thisinvention isIOnyxide which is an oleyl dimethyl ethyl ammonium bromide. The non-ionic surfactants. which are efiective may be classified as esters of anhydro alkitols and fatty acids as described in US. 2,398,193. The particular examples suitable as surfactants of this invention are known com mercially as the Spans and Arlacels. The Span materials are essentially partial esters of common fatty acids (lauric, palmitic, stearic and 'oleic) and hexitol anhydrides (hexatans and hexides) derived from sorbitol. Sorbitol is dehydrated to give condensed ring structures of hexitans and hexides which when esterified with fatty acids produces the Spans or Arlacels. Arlacel C, which is sorbitan sesquioleate, is an oily liquid at 25 C. having a specific gravity of 0.9Sl.00, a flash point of about 450 F. and a fire point of about 530 F. and is prepared by treating the Span product to obtain a product of light color, i.e., the chemical structure of Arlacel is similar to that of the corresponding Span product. Arlacel C was found to be effective as a surfactant agent in the milling of the gas-forming compositions of this invention.

Another type of non-ionic surfactant which I have found useful in the compounding of the improved gasforming compositions are the polyethylene oxide ethers, an example of which is the commercial product, Tergitol Dispersant XC, a semi-waxy solid polyethylene oxidepolypropylene oxide ether, having a melting point of 34-36 C., a flash point of 475 F., a fire point of 580 F. and a viscosity at 55 C. of 308-310 centistokes and a molecular weight of about 3000.

In preparing the compositions of this invention, the binder material is first prepared and the ammonium nitrate, thoroughly mixed with the catalyst, is milled with the plasticized binder. If carbon is included in the composition it is mixed with the ammonium nitrate as is also the surfactant when used in the composition. The binder is prepared by heating the plasticizer material at a temperature not in excess of about 150 C., usually within the range of from about 120 C. to about 140 C. The heated plasticizer material is stirred and the polyvinyl chloride is added, heating and mixing being continued until a homogeneous mixture is obtained. The temperature of the plasticized polyvinyl chloride is then permitted to lower below about 120 C. and preferably to a temperature not below about 100 C. and the ammonium nitrate with the catalyst is added to the binder material. The mixture is milled until a product of uniform texture is obtained. Burning rate test strips may be extruded or molded under pressure. The rate of burning of the grain material is determined at different pressures in an inert atmosphere.

Preparation and testing of burning rate test strips Burning rate test strips of the gas-forming composition are prepared by extruding or molding the homogeneous gas-forming composition at a temperature below about 120 C. under pressure of about 2000 p.s.i., the test strips being A" to in diameter and about 5" long. In preparing the test strips by extrusion, the material is placed in a 1" diameter cylinder and is subjected to about 2000 pounds pressure and either extruded by a screw or by a ram through an aperture of diameter for ram extrusion or about A" diameter for screw extrusion. After cooling, the strand is cut into test strips approximately 5" long and these are then coated with lacquer grade cellulose acetate to inhibit surface burning along the strand in the burning test. In preparing the test strips by molding, the formulated composition is molded at 2000 p.s.i., into rectangular strips of about 1" x A" cross-sectional dimension. These strips are then cut into 12 x test strips about 5" in length for use in determining burning rates. The strips, extruded or molded, are provided with drilled holes 3" apart through which are passed fusible wires which are connected to a timing device for obtaining the burning rate. After coating the strips with lacquer grade cellulose acetate, the test piece is placed in a pressure bomb and electrical connection of the fuse wires is made to 8 the timing device. The timing device is started by fusing of one wire and as the test piece burns along its length the timing device is stopped by the fusing of the second wire. Thus the time for burning of the 3" of the test piece is obtained. The test piece is ignited by means of a Nichrome resistance wire placed in contact with one end of the test piece. Burning rates for the test pieces are determined at pressures of 600, 800, 1000, 1200, 1400, 1600 and 1800 pounds per square inch nitrogen pressure. Burning rates of compositions of the examples below, unless otherwise specified, are burning rates at 1000 p.s.i.

The burning rate in inches per second for the different pressures are plotted on log-log paper and the plot gives a straight-line relationship, the burning rates being plotted vertically and the pressures being plotted horizontally. The slope of this straight line is defined as the exponent of the burning rate as related to pressure in the formula where B is the linear burning rate at pressure p," 6 is the linear burning rate for the composition at 1000 p.s.i., p is pressure in p.s.i. in the burning chamber and n is the pressure exponent showing dependence of the burning rate on pressure. Thus the exponent is the numerical value equal to the slope of the curve of burning rate in inches per second versus pressure obtained by plotting the burning rate at various pressures on log-log paper. This relationship of burning rate and pressure at constant temperature was developed by R. N. Wimpress in Internal Ballistics of Solid Fuel Rockets (1950). Ammonium nitrate compositions usually have a pressure exponent of about 0.7 or higher. The lower the value of n the less is the detonating character of the decomposition of a gas-producing composition and the more even and smooth is the burning rate of the propellant grain. Thus a sustained thrust rather than a detonation is produced in the burning of the propellant grain and a sustained fiow of gas from the gas generator is obtained if the pressure exponent of the composition is low. A characteristic of-binder material containing polyvinyl chloride as plasticized polymeric base material is that satisfactory smooth burning rates of the ammonium nitrate grain are obtained without the addition of material to lower the slope of the pressure exponent curve.

Gas-producing grains are prepared from the above compositions containing ammonium nitrate, combustion catalyst and binder material by molding the compositions into cylindrical grains under a pressure of about 2000 to 4000 p.s.i. The size and shape of the grains are dependent upon their intended use. Grains for airplane assist take-off service are usually about 30" in length by 3" to 6" in diameter. These are provided with centrally located holes of diflerent shapes, that is, starform, cruciform or circular. Test grains subjected to tests described hereinbelow and for use in a test motor, which grains were 2.75" in diameter and about 5" in length, were provided with a starform centrally located hole. The test data obtained by burning such grains in the test motor indicate the thrust or impulse uniformity of burning rate and over-all performance of the compositions when used in assist take-off operations. For these operations the grains are mounted in'a conventional case and are ignited or fired by electrical or other known means. The temperature of the composition gases produced by firing of the grain may be of the order of 1500 F. to 3000 F. and the pressure or impulse produced by the hot gases will be dependent upon the grain size, diameter of the nozzle and other factors. The gas-. producing grains may be molded into discrformystaeks of discs being used as gas-forming propellant'material for missile rockets. e s

As indicated hereinabove, the testgrains, that is, the grains of diameter 2.75" by in length "are given a thermo shock test which is referred to herein. as the thermo cycling test or simply as the cycling test. In this test, several of the grains of a given composition are subjected, in an oven, to a temperature of 175 F: for a period of two hours, following which the grains are immediately subjected for a period of two hours to a temperature of 75 F. to complete one cycle, immediately after which the grains are again subjected to the 175 F. temperature for a period of two hours and to a second cold temperature of 75 Flfor two hours after which at least one of the grains is permitted to come to normal room temperature, i.e., about 75 to 80 F. This or these grains are examined for indications of shattering, crystallinity and resistance to deformation and shattering when dropped a distance of at least 5 on a concrete surface. Part of the grains are tested for firing qualities, the temperature of the grains being 75 F., 175 F. and normal ambient temperatures following the cycle treatment. In general, the grains made up with the plasticized polyvinyl chloride of this invention show no physical defects following the v thermo cycling test. I

Another test to which the finished grains are subjected is that of hot aging. In this test, the grainsare maintained at a temperature of 170 F. in an oven in the presence of circulating air. Several grains of a given composition are used in a test, grains being withdrawn periodically, which are tested with respect-to firing and burning qualities. In order to pass this test, a grain must be unaffected, with respect to; firing and burning qualities at the end of 30 days ofsuch high temperature treatment. A grain which has been made defective by such treatment usually fires and burns. withcxplosive violence rather than normal nonexplosive firing and uniform burning of the grain. In general, .theamrnonium nitrate grains made up with. the plasticized polyvinyl chloride of this invention are superior with respect to resistance to firing and burning defectsi asproduced in this test. 7 a Another requirement which is met by the ammonium nitrate-binder compositions of this invention is that of chemical stability at relatively high temperatures as shown by the substantial absence of gassing of the composition.

The gassing tendency is measured by an; arbitrary test' which has been made more severe with respect totemperature than would be imposed on the composition at atmospheric temperatures. This test is commonly designated as 135 C. Gas Stability Tes This test is'carried out as follows:

Three grams of a finely divided ammoniumnit-rate base composition is placed in a vessel. nected by tubing to a mercury manometer system which The vessel-is con- 3 is so arranged that differential readings of the manometer 1 are translatable into volume changes in the system. Since the volume change of the sample itself can be disregarded,

amount of gaseous decomposition products of the sample. The vessel is inserted to an opening in a metalblock: this metal block is provided with electrical heating..elements and with controls which permit the. block to -be maintained at a temperature of 135 C. Aperiod of 15 minutes is allowed for the sample to come tothetempera- .ture of 135 C. At this time the manometer is zeroed. The readings are taken at 15-minute intervals .until sufficient readings have been taken to indicate that the gas product rate is substantially constant, that is, has reached an equilibrium value or that the gas product rateis so low that the sample obviously has more than satisfactory/low directly to cc./g./hr. for each 15 minute interval. The

'60 the volume change in the system corresponds to the "grade Geon 1012 10 gas rate in cc:/ g.'/hr. for each 15 minute interval is then plotted against the time of heating of the sample. This rate is referred'to as cc./g./hr. s When the heating is continued for an appreciable length of time, the gassing rate is referred to as the gassing rate at the end of one hour, two hour and three-hour periods of heating and also the maximum rate of gassing and the time at which the maximum rate is reached is recorded.

In general, it is considered that a composition. which has a zero gassing rate during the first hour of heating (essentially zero means within the limit of error of the test which is of the orderof 0.3'cc./g./hr.) will be substantially free of gassing tendency in storage atatmospheric temperatures. It is considered that a composition which has a gassing tendency not in excess of about 2 cc./g./hr. at the first hour of heating at a temperature of 135- C. has a satisfactory gassingtendency with re-.

spect to hot storage, i.e., at'temperatures as high as 77 7 C. for a period of 30 days. The compositions of this in vent-ion consisting of the defined binder, ammonium nitrate and inorganic catalyst exhibit extremely low gassing properties in this test, and are satisfactory without the addition of gassing tendency inhibiter.- Even the presence of finely divided carbon in the composition, in gen-. eral, does not render the composition unsatisfactorywith respect to storage stability at atmospheric temperatures.

The test series described below are illustrative of the method of compounding the compositions of this invention and of the properties and performance of grain.compositions and shaped grains with respect to burning properties after being subjected to extremely high and low temperatures. In general, the compositions show very little tendency to gas appreciably when subjected to a temperature of 135 C. for several hours and as indicated hereinabove the shaped grains are resistant to shattering or in ternal fissuring when subjected to physical shock.

The ammonium nitrate used in the test series was commercial grade, 70% of the nitrate incorporated in. the compositions being ground to finely divided powder, i.e.,

to pass through a #100 US. Standard mesh screen,

the remaining 30% of the unground nitrate of the compositions being used in the unground conditions to prevent caking, and thereby to facilitate better mixing. The ammonium dichromate catalyst used was commercial grade ammonium dichromate and the Prussian blue catalyst, where used, was of the insoluble Prussian blue type. The polyvinyl chloride used was the commercial 7 *Test Series A Abinder material was prepared by heating at about 140 C., 7.87 parts by weight of 2,4-dinitrodiphenyl ether in admixture with 7.88 parts by weight of commercial grade 2,4-dinit-rotoluene. To the molten-mixed plasticizer was added 5.25 parts by weight of polyvinyl chloride and the mixture 1 was stirreduntil a homogeneous plasticized material was obtained. To this plasticized mass was added til par-t by weight of Tergitol Dispersant XC surfactant and the temperature of the mixture was lowered toabout 120 C; A mixture containing 75.9 parts by weight of ammonium nitrate, 2 parts by weight of ammoniumdichromate catalyst and 1 part by weight of insoluble Prussian blue catalyst was then mixed and milled into the binder until a uniform homogeneous product was obtained. Burning rate strands were produced from this composition by ram extrusion and 'shapedgrains were produced by molding under .pressure as described hereinabove. The burning rate of the'strands at 1000 p.s.i. was found to be 0.165 and the pressure exponent of the composition was 0.58. Shaped grains produced from the composition were cycled at successive temperatures of 175 jFvand -75 F. It was" found that the:

grain would ignite at the 75 F. temperature and at ambient temperature,.i.e., at about F. following the cycles, and that the grain burns uniformly following the cycles, i.e., internal fissuring did not result from the shockt'emperature treatment. Shaped grains of this composition were also subjected to 170 F. temperature in the aging test and grains fired normally after being held for a period of 100 days at 170 F. The composition showed satisfactory properties with respect to gassing and after being subjected for several hours to a temperature of 135 C.

Propellant grain material was prepared in the same manner and from the same components as described above in this Test Series A except that no surfactant was added. The composition of the grain on a weight basis, was 76% ammonium nitrate, 5.25% polyvinyl chloride, 7.87% 2,4-dinitrodiphenyl ether, 7.88% 2,4-dinitrotoluene and as catalyst 2.0% ammonium dichromate and 1.0% of insoluble Prussian blue. Burning rate strands were produced by molding as described hereinabove. The burning rate of the strands at 1000 psi. was 0.126 inch per second and the pressure exponent of the burning rate was 0.56. The lower burning rate was due to the method of fabricating the burning rate strands, i.e., molded strands give lower burning rates which more closely approximate the burning rates of molded grains when burned in the test motor. Following thermal shock treatment, i.e., successive subjection to 175 F. and 75 F., the grains prepared from this material showed firing and burning properties no different than similar grains containing the surfactant.

A third composition was prepared in the same manner as the above composition containing the Tergitol XC except that Onyxide was used as a surfactant, the other components of the composition being the same and in the same relative proportions. Substantially the same properties for this grain composition were observed. The burning rate at 1000 pounds p.s.i. was 0.160. The grain showed burning properties unafiected by the shock-temperature treatment in the cycling test, passed the 30-day hot storage test and the composition showed little tendency to gas at 130 C.

In another experiment, a gas-forming propellant composition was prepared using the same polyvinyl chloride, the dichromate-Prussian blue catalyst ammonium nitrate,

at 170 F. and the material showed low gassing tendency at 135 C. In still another test this composition containing 9.26 parts by weight of 2,4-dinitrodiphenyl ether and 6.49 parts by weight of dinitrotoluene, the other components of the composition being the same. The buming rate was enhanced by incorporating with the ammonium nitrate 1 part by weight of carbon black in the form of Micronex Beads. Burning rate at 1000 pounds p.s.i. of a ram extruded strand of this composition was 0.185 and the pressure exponent was 0.70. Thus, the substitution of a relatively small part of the ammonium nitrate with the carbon black, that is, suflicient to give 1% carbon black in the total composition, raised the burning rate somewhat more than 20%. This was accomplished without aflecting the properties of grains produced therefrom which were subjected to the shockthermo treatment (cycling), hot aging (170 F. for 30 days) and the gassing tendency of the composition remained satisfactory.

In still another experiment of this series, a composition consisting of the same combustible materials as the above compositions was formulated, the only difference being that the composition contained no ammonium dichromate catalyst. The catalyst used was insoluble Prussian blue and the composition contained carbon black as a burning rate synergist. The composition consisted of 5.62% polyvinyl chloride, 8.44% dinitrodiphenyl ether, 8.44% dinitrotoluene, 3% insoluble Prussian blue, 1% carbon black (Micronex Beads), the remaining 73.5% of the composition consisting of the partially pulverized ammonium nitrate described above. The burning rate of a ram extruded strand of this material was 0.175 and the pressure exponent was 0.70.

The elfect of using the less active insoluble Prussian blue catalyst was to lower the burning rate below that obtained when a mixture of the ammonium dichromate and insoluble Prussian blue catalyst was used.

The burning properties of compositions containing nitro compound plasticizer components other than and in addition to dinitrotoluene and dinitrodiphenyl ether are shown in the table below. These compositions were prepared as described above in Test Series A.

TABLE Ammo- Other PVC, Plasticlzer, Percent Catalyst, nium Components, 13.12. Pressure Percent Percent lil'itrate Percent Exponent ercen 5.0 2,4-dlnltrodiphenyl ether, 8.0; 2,4di- A,2.0

nitrophenyl allyl ether, 8.0. {3, 1.0-.- i 0 X0 19 4.25 2,4 d initrotoluene, 4.25;m-dinitroben- A 20 1 e, 4.25; 2,4-dlnitrodlphenyl ether, 79.8 'r, xo 0.2--- 0.10 0. 12

5.25 2,4-dinitrophenyl propyl ether. 7.88; 11,2.0.

2,4-dlnitrophenyl methyl-ether, 7.87. {13, 1.0. M9 3.0 2,4dinitrophenyl allyl ether, 3.0; 2,4 B, 3 83.0 MB 2.0 0.165 0.86

dinltrotoluene, 2.4; 2,4-dlnitrodiphenyl ether, 3.6. 3.75. 2,4-dlnitrophenyl allyl ether, 3.75; 2,4- B,3 82.0 None 0.15 0.76

dlnitrotoluene, 3.00; 2,4-dinitrodlphenyl ether, 4.50.

A=Ammonium dichromate. B=Insoluble Prussian blue.

Tergitol Dispersant XC surfactant, and 2,4-dinitrodiphenyl ether-dinitrotoluene plasticizer as except that the ratio of 2,4-dinitrodiphenyl ether to dinitrotoluene in the plasticizer was changed from 1 to l to 1.43 to 1, the ratio of total plasticizer to polyvinyl chloride and binder material to ammonium nitrate being the same as above, that is, 9.26 parts by weight of the 2,4-dinitrodiphenyl ether and 6.49 part-s by weight of dinitrotoluene were used as the plasticizer. The burning rate of this composition at 1000 p.s.i., was also about 0.16 and shaped grains produced from the composition showed. satisfactory burning after cycling and hot aging The results shown in the table indicate that such nitro described above, 65 aromatic compounds as dinitrophenyl propyl ether, di-

nitrophenyl methyl ether and dinitrophenyl allyl ether may be used as plasticizers for the fabrication of satisfactory grain compositions, eitherin the absence or presence of a surfactant such as Tergitol X0 or in the presence or absence of finely divided carbon such as Micronex Beads.

The effect of including, in the plasticizer of the gas- :forming composition, a compound containing no, nitro substitution group was examined in another series of tests. However, the plasticizer components of the compositions included a major proportion of those nitro substituted 13 compounds used in Test Series A, above. The results obtained indicate that non-nitro substituted plasticizers may be used in minor proportions and they areypreferably used in amounts less than about 40% by weight of the total plasticizer components. Ye a Test Series A mixture of plasticizer material consisting of 4.77 parts by weight of 2,4-dinitrotoluene, 4.70 parts by weight of 2,4-dinitrophenyl allyl ether, 4.78 parts by weight of 2,4-dinitrodiphenyl ether and 2.2 parts by weight of Plastoflex MGB was heated to a temperature of about 140 C. to obtain a homogeneous mol-ten mixture. To this mixture was added 5.5 parts by weight of polyvinyl chloride (Geon 101). The resulting binder material was stirred to obtain homogeneous plasticized polyvinyl chloride. The temperature was lowered to about 120 C. and a mixture consisting of 74.9 parts by weight of ammonium nitrate, and 3 parts by weight of insoluble Prussian blue catalyst was mixed and milled with the binder. Tergitol XC surfactant (0.1% by weight of the total gasforming composition) was added to the mixture during this final mixing operation. A part of the mixture was extruded into burning rate strands and a part was shaped into grain material. The strands showed a burning rate at 1000 p.s.i. of 0.14 inch per second and a pressure exponent of 0.77. The grains were cycled at high and low temperatures and were fired successfully after being subjected to the thermal shock in the cycling test. The grains also passed the 30-day hot aging (170 F.) test and they showed very little tendency to gas when subjected to a temperature of 135 C. for several hours.

A gas-forming composition similar to the above Plastoflex MGB-containing composition was prepared in the same manner and from the same components except that carbon black in the form of Micronex Beads was included and mixed ammonium dichromate-Prussian blue catalyst was used in the composition. This composition also consisted of 4.75% by weight of polyvinyl chloride, plasticized with a plasticizer consisting of 4.12% by weight of dinitrotoluene, 4.10% by weight of the dinitrophenyl allyl ether, 4.13% by weight of the dinitrodiphenyl ether and 1.90% by weight of the Plastoflex MGB, the percents by weight being based on the total weight of the composition. The composition contained 76.8% by weight of ammonium nitrate, 2% by weight of ammonium dichromate catalyst, 1% Prussian blue catalyst, 1% by weight of Micronex Beads and 0.2% of Tergitol XC surfactant. Strand material formed from this composition showed a burning rate at 1000 p.s.i. of 0.175 inch per second and a pressure exponent of 0.63. Grains which were shaped from the composition were fired successfully after cycling and after aging for 30 days at 170 F.

In another test of this series, a grain compositionvcontaining 5.22% of polyvinyl chloride (Geon 101), 15.69% of 2,4-dinitrodiphenyl ether and 2.09% of dioctyl phthalate plasticizer, 75% of ammonium nitrate and 2% of insoluble Prussian blue was formulated according to the procedure described above. The burning rate at 1000 p.s.i. of a ram extruded strand of this material was 0.145 and the pressure exponent was 0.71. When subjected to thermal shock in the cycling test, this material showed no evidence of development of internal fissures or external cracking or checking of the surface. In still another test, a gas-forming composition consisting of 5.25% of polyvinyl chloride (Geon 101), 13.65% of 2,4- dinitrodiphenyl ether and 2.10% Plastofiex MGB plasticizer, 75.9% ammonium nitrate and 0.1% Tergitol XC surfactant was formulated. This material showed a burning rate at 1000 p.s.i. of 0.162 and grains shaped from the material showed satisfactory firing and burning following thermal shock treatmentin thecycle test and by weight of' also showed satisfactory firing after being subjected to hot aging at 170 F. for a period of 30 days. a

Havin thus described the invention what I claim is:

1. A gas-forming composition comprising about 73% to about 88% 'by weight ofammonium nitrate, from about 1% to about 5% by weight of'a combustion catalyst selected from the-class consisting ot Prussianblue and ammonium dichnomate and from about 10%. to

about 25% by weight of a plastic binder said binder con sisting essentially of about 15% to about 35% by weight of polyvinyl chloride and from about 65% to about by weight of a plasticizer, said plasticizer comprising from about 60% to about of at least one dinitrosubstituted phenyl nucleus-containing aromatic compound corresponding to the general formula the polyvinyl chloride is characterized by a 0.4% solution of said polyvinyl chloride in nitrobenzene having a specific viscosity at 20 C. within the range of from about 0.50 to about 0.60.

3. The composition as described in claim 1 wherein the inorganic catalyst consists of a mixture of insoluble Prussian blue and ammonium dichromate.

4. The composition as described in claim 1 containing from about 0.05% to about 0.5% by weight of a surfactant agent consisting essentially of polyethylene oxide, polypropylene oxide ether having a molecular weight of about 3000.

5. The composition of claim 1 to which is added from about 1% to about 5% of finely divided carbon.

6. The composition as described in claim 1 wherein the inorganic catalyst consists essentially of insoluble Prussian blue.

7. A shaped gas-forming grain consisting essentially, on a weight basis, of

Percent Ammonium nitrate 76.00 Ammonium dichromate 2.00 Insoluble Prussian blue 1.00

Polyvinyl chloride of specific viscosity of 0.52 to 0.58 5.25 2,4-dinitrodiphenyl ether 7.87 2,4-dinitrotoluene 7.88

'8. A gas-forming composition comprising about 73% to about 88% by weight of ammonium nitrate, from about 1% to about 5% by weight of a combustion catalyst selected from the class consisting of Prussian blue and ammonium dichromate and from about 10% to about alyst selected from the class consisting of Prussian blue and ammonium dichromate and from about 10% to about 25% by weight of a plastic binder said binder consisting 15 essentially of about 15% to about 35% by weight of a polyvinyl chloride and from about 65% to about 85% by weight of a plasticizer, wherein the plasticizer consists of a mixture of dinitrodiphenyl ether and dinitrophenyl aHyl ether.

10. A gas-forming composition comprising about 73% to about 88% by weight of ammonium nitrate, from about 1% to about 5% by weight of a combustion catalyst selected from the class consisting of Prussian blue and ammonium dichromate and from about to about 25% by weight of a plastic binder said binder consisting essentially of about to about by weight of a polyvinyl chloride and from about to about by weight of a plasticizer, wherein the plasticizer consists essentially of a mixture of a dinitrotoluene, a dinitrodiphenyl ether and a dinitrophenyl allyl ether.

No references cited. 

1. A GAS-FORMING COMPOSITION COMPRISING ABOUT 73% TO ABOUT 88% BLY WEIGHT OF AMMONIUM NITRATE, FROM ABOUT 1% TO ABOUT 5% BY WEIGHT OF A COMBUSTION CATALYST SELECTED FROM THE CLASS CONSISTING OF PRUSSIAN BLUE AND AMMONIUM DICROMALTE AND FROM ABOUT 10% TO ABOUT 25% BY WEIGHT OF A PLASTIC BINDER SAID BINDER CONSISTING ESSENTIALLY OF ABOUT 15% TO ABOUT 35% BY WEIGHT OF POLYVINYL CHLORIDE AND FROM ABOUT 65% TO ABOUT 85% BY WEIGHT OF A PLASTICIZER, SAID PLASTICIZER COMPRISING FROM ABOUT 60% TO ABOUT 100% OF AT LEAST ONE DINITROSUBSTITUTED PHENYL NUCLEUS-CONTAINING AROMATIC COMPOUND CORRESPONDING TO THE GENERAL FORMULA 